1. Field of the Invention
The present invention relates to the inspection of nuclear fuel rods, and assemblies of fuel rods, to detect and locate defective rods.
2. Related Art
The large nuclear reactors utilized for power generation employ an array of a large number of fuel rods containing nuclear fuel. Each rod comprises a metal tube or sheath which may be from 8 to 15 feet long and approximately ½ inch in diameter, and which contains a stack of cylindrical fuel pellets of suitable fissionable material such as uranium oxide. The upper end of the tube is empty of fuel pellets and forms a plenum for a gas or other fluid under substantial pressure which fills the top of the rod and also a small clearance space which is provided around the fuel pellets to allow for expansion or swelling as a result of irradiation. The fuel rods are supported in parallel groups in fuel assemblies which may typically contain upwards of 300 fuel rods, and the complete nuclear reactor is made up of a large number of these fuel assemblies arranged in a suitable configuration in an active core.
The metal tubes of the fuel rods, also known as cladding, constitute the primary containment boundary for the radioactive nuclear fuel, and inspection to verify the integrity of the rods is of primary importance. In the manufacture of the fuel rods, the tubing itself and the end cap welds are carefully inspected and helium leak tested. Since a nuclear reactor may contain upwards of 40,000 fuel rods, a probability exists that some number of defective tubes will be present even with a highly effective manufacturing quality control program. Furthermore, even initially good fuel rods may develop cracks, pinholes or other defects in service and such defective rods must be detected.
The reactor is usually shut down approximately every 12 to 24 months for refueling. During the refueling outage, as well as during initial installation, the fuel rods must be inspected to detect any defective rods that may be leaking fission products. The reactor and the fuel assemblies are immersed in a pool of water during the refueling operations and during removal of the fuel assemblies for replacement or inspection. In the standard method of inspection known as sipping, which has been used heretofore, the flow of water through each fuel assembly to be inspected is blocked so as to allow the fuel rods to heat up, which causes expulsion of fission products into the water through any defects that may exist in the rods. The water is then checked to detect the presence of radioactivity, indicating that such a leakage of fission products has occurred and that a defective rod is present in the assembly being tested. This system requires that the water be pumped to a sampling station and repeatedly analyzed or checked for radioactivity and is a very slow procedure requiring as much as an hour for each assembly. Complete inspection of all fuel assemblies in a large power reactor is, therefore, very time consuming during which the reactor is out of service.
Fission products leaking from failed fuel assemblies can cause many conditions that increase operating costs. These conditions include (1) high radiation readings in the primary coolant system; (2) increased volume of liquid radioactive waste; (3) increased volume of solid radioactive waste due to more frequent demineralizer bed replacement; (4) increased costs for disposal of spent fuel assemblies due to special handling and additional decontamination; and (5) increased exposure to personnel. These increased costs outweigh the costs incurred by testing the assemblies. Currently, the fuel assemblies are tested using fuel “sipping” as mentioned above. A positive finding indicates the presence of a leaking rod, or rods, within an assembly but does not pinpoint the specific culprit. To narrow the leakage to a specific rod, sipping is supplemented by an ultrasonic technique that looks for evidence of water inside the individual rods. Once identified, the leaking fuel rod may be extracted from the assembly and replaced with a dummy rod to allow the eventual reload of the assembly in the core. If identification of the specific degradation mechanism that resulted in the leakage is desired, the assembly can be prepared so that the rods can be extracted and a variety of inspections conducted on the individual rods. Individual rod inspections include high resolution visual inspection, eddy current inspection and ultrasonic inspection, looking for discontinuities in the cladding rather than the presence of water. While the time requirements for sipping and ultrasonic testing for a single assembly does not seem significant, the time for testing an entire core is approximately two days. With increasing pressure to decrease outage durations, the nuclear industry will benefit immensely from a leak detection system that would reduce the time required to locate leaking assemblies and especially leaking fuel rods.